1. Field of the Invention
The present invention is in the field of superplastic forming of metallic workpieces and, more particularly, to workpieces composed of aluminum.
2. Description of the Prior Art
Superplastic metals are known in the art as materials that provide the strength of conventional metals and the elongation and formability characteristics of conventional plastic materials. However, at superplastic forming temperatures, the metallic workpieces may warp and/or distort if removed from the die or otherwise handled without proper support. Thus, prior art devices and superplastic forming systems typically require that the metallic workpiece be allowed to cool before handling or removal from the die.
Many prior art systems seek to facilitate removal of the workpieces from the die by making the die with a relatively high degree of draft. The draft tends to prevent the workpiece from fitting too snugly within the die cavity causing binding at some of the edges or corners and impeding removal therefrom. Without a draft, the workpiece would have to be pulled straight up out of the die with the sides of the workpiece in sliding contact with the sides of the die. Thus, a relatively high degree of draft allows the part to be more easily slipped out of the die cavity. However, using such a high degree of draft in the die necessarily places important limitations on the shape of the die and on the shape that the die may impart to the workpiece.
Another prior art system incorporates a two piece die one of which is provided with an aperture. After a workpiece is superplastically formed in the die cavity between the pieces, a vacuum is applied at the aperture to draw the workpiece up against one of the die halves. The vacuum holds the workpiece against the die half so that the upper die half can be lifted from the lower die half carrying the workpiece with it. However, the disadvantage with this system is that support is not provided at all points of the workpiece surface adjacent the upper die half. Moreover, the entire upper die half and workpiece must be cooled before the die can be reused to superplastically form another workpiece. Two examples of such a prior art system are disclosed in U.S. Pat. Nos. 4,381,657 and 4,502,309 both to Hamilton.
Other prior art devices have used suitably positioned pins to eject the workpiece from the die after completion of the superplastic forming process. The pins are mounted in the die and abut the workpiece. When actuated, the pins push the workpiece away from the die. However, a major disadvantage with this prior art system is that if the pins are actuated when the workpiece is too hot, the pins may puncture or otherwise damage the workpiece. In addition, with such systems the workpiece must be of a sufficient thickness to withstand the force imparted by the moving pins. Consequently, due to these limitations in use, the pin ejection system is only of marginal benefit in removing a workpiece from a die.
Still other prior art systems have attempted to facilitate removal of the workpiece from the die by positioning the die member containing the cavity upside down. Thus, at the end of the superplastic forming process when the lower die member is separated from the upper member, the workpiece can simply fall out of the die cavity. As with the other prior art devices described herein above, this system must also allow the workpiece to cool before separation from the die in order to avoid warpage and/or damage to the workpiece caused by excessive handling or premature removal from the die. Thus, since the workpiece must be allowed to cool before removal, the production time for superplastical forming using this prior art system is inordinately long.
In conclusion, since support is needed for the workpiece while it is still in a superplastic state to prevent warpage and distortion, prior art systems currently in use typically require the superplastically formed part to cool to a substantial degree while still within the die. Consequently, using prior art systems, production of superplastically formed parts requires that the entire die and workpiece together be allowed to cool down before the workpiece may be handled separate therefrom. The cooling down period may be inordinately long if the die and workpiece are very large and hold a lot of heat energy. Thus this required cooling off period results in relatively long production times required to superplastically form a desired part.